Semiconductor die, such as vertical conduction power switching die like MOSFETs, have been packaged as SO8 device packages using wirebonds. For example, in the case of a vertical conduction power MOSFET having a drain electrode on one surface of the die and source and gate electrodes on an opposing surface of the die, an SO8 device package may have the drain electrode directly attached through a conductive adhesive to a die pad of the package lead frame, which die pad may be integral with one or more terminal leads. In turn, the source and gate electrodes on the top surface of the die may be electrically connected through wirebonds to one or more respective terminal leads. Notably, it has been found that the wirebonds that connect the source electrode to the package terminal leads increase the resistance of the overall device package.
To reduce this package resistance, wirebonded SO8 device packages described above are being gradually replaced with device packages that use a copper strap to electrically connect the source electrode to the terminal leads. Such copper strap device packages have taken various forms, including SO8, MLP and QFN device packages.
A known SO8 device package 100 is shown in FIGS 1A and 1B. It should be noted that the device package shown in FIG. 1A has protective housing 114 removed. Device package 100 has a lead frame that includes a die pad 118 and a plurality of terminal leads 120-127. As shown, terminal leads 124-127 are integral with die pad 118 and terminal leads 120-122 are integral with one another. Device package 100 also includes a power switching die 102, such as a power MOSFET, having a source electrode 104 and gate electrode 106 on a top surface thereof and a drain electrode 108 on a bottom surface thereof. While source electrode 104 is shown as a single electrode, it may also be formed as two or more electrodes.
Similarly to the wirebonded SO8 device package described above, die 102 is mounted within package 100 such that drain electrode 108 is directly attached through a conductive adhesive, such as solder or conductive epoxy, to die pad 118. Similarly, gate electrode 106 is electrically connected through wirebond 112 to the bond pad of terminal lead 123. However, rather than connecting source electrode 104 through wirebonds to the terminal leads, source electrode 104 is now connected through a copper strap or copper clip 110 to the bond pad of terminal leads 120-122. Device package 100 also includes protective housing 114 formed of a moulding compound that covers at least copper strap 110, die 102, and a portion of the lead frame.
In general, it has been found that device packages (e.g., the SO8, MLP and QFN packages) using a copper strap to electrically connect the source electrode to the terminal leads have a lower package resistance than wirebonded SO8 device packages. In addition, the packaging costs of the copper strap device packages also have packaging costs that are not more than the wirebonded SO8 device package. Accordingly, the copper strap device packages are becoming increasingly popular and are being used to gradually replace the wirebonded SO8 device package.
As shown in FIGS. 1A and 1B, the copper straps, such as strap 110, used in the copper strap device packages generally include a contact 110a that is electrically connected through a conductive adhesive, such as solder or conductive epoxy, to the die electrode, include a leg 110c that is electrically connected through a conductive adhesive to the lead frame, and include a web 110b that connects the contact to the leg. In general, among other features, it is desirable to improve the solder joints between the die electrode and contact 110a and between the lead frame and leg 110c, thereby improving copper strap device packages, such as copper strap SO8, MLP, and QFN device packages.